In the art of non-impact printing it is known to selectively activate recording elements such as LEDs or a laser to control exposure of a recording medium. In order to activate the recording element(s) a driving system is provided to control exposure time and intensity of a selected recording element for recording each pixel.
A hybrid driving system featuring modulation of both the exposure time and the exposure intensity are often used as either LED writers and laser printing systems to extend the exposure dynamic range of an imaging system (compared to when only exposure modulation is used or pulsewidth modulation (PWM) is used) and have the potential to create many levels of grey at the same time (so as to reduce density contouring). Cases for PWM, intensity modulation and hybrid modulation and their resultant exposure are shown in FIGS. 1, 2 and 3 respectively. However, due to system non-linearity and noise in measurement (when we characterize the exposure in intensity and exposure time) and cross-talk problems in actual usage (such as caused by current starvation), switching from one intensity level to the higher intensity level while lowering the exposure time to give you the next gradation does not necessarily generate the desired result of density increase. So instead of the density resulting from the exposure continuing to go up, it may actually go down (or there is a discontinuity in density direction of change) at the boundary of the intensity and exposure time switching. The result is some density contouring or some noticeable density discontinuity at those switchover points in printing (see FIG. 4). In more detail, we have shown in the table of FIG. 5 that between events 10 and 11 when the rise and fall time effect is there (in case 2 for a practical case) versus a theoretical estimate (in case 1), the switching of intensity of the hybrid approach can cause in the practical case a density reversal (even though the intensity change was accurate). This is the case since rise and fall time effect has more impact on shorter exposure times than longer exposure times. In another situation, such as events 30 and 31 when there is a slight intensity estimation error (either due to a temperature effect or brightness/drive current non-linearity), density reversal can occur. In this case, both the rise/fall time effect and intensity estimation error is included. These events are shown as density reversal at events 11 and 31 in FIG. 5. One may have noticed that the problem of density reversal would not occur if only pulsewidth or intensity is changing by itself monotonically, i.e. changing in one direction only.